1. Field of the Invention
The present invention relates to a CMP (Chemical-Mechanical Polishing) pressure monitoring system. More particularly, the present invention relates to a close loop pressure monitoring system for regulating air streams used in operating chemical-mechanical polishing process on a real-time basis.
2. Description of the Prior Art
Recently, chemical-mechanical polishing (CMP) processes have become very well received in the planarization of a wafer surface in preparation for further device fabrication. The CMP process mainly involves holding a semiconductor wafer against a rotating polishing pad surface wetted by a polishing slurry, which typically comprises an acidic or basic etching solution in combination with alumina or silica particles. On one hand, the liquid portion of the slurry chemically removes, loosens, or modifies the composition of the material on the wafer which is to be removed. On the other hand, the particle portion of the slurry, in combination of the rotating polishing pad, physically removes the chemical modified material from the wafer. Thus, the name chemical-mechanical polishing was obtained.
A typical CMP set-up is demonstrated in FIG. 1A. Semiconductor wafer 10 is mounted with the surface to be polished exposed, on a wafer carrier 12 which is part of (or attached to) a polishing head 14. The mounted substrate is placed against rotating polishing pad 20 disposed on a base portion 22 of the polishing machine 24. In addition to the pad rotation, the carrier head 14 may rotate to provide additional motion between the substrate 10 and polishing pad 20 surface. The head and base portions of the polishing machine are generally equipped with pneumatic conveyor lines for supplying air forces (wafer air 30 and polish air 40 respectively) against the backsides of the substrate and polishing pad. The pressures of the air streams, designated as wafer air 30 and polish air 40 in the figure, are individually adjusted to secure the polishing surfaces in an appropriate contact. These air pressures contribute main sources of mechanical force during polishing operation and thus are an important factor that can affect the result of the CMP process.
Referring to FIG. 1B, the CMP structure is disclosed. One way used to control the air stream pressure values has been to put a pressure transducer 50 in each of the air flow lines to independently set the air pressures for each of the different air streams. A transducer 50 is incorporated with each regulator 52 to generate a feedback signal proportional to the pressure of the air in the outlet port of the air regulator. Necessary adjustments of the corresponding transducers 50 are made so as to maintain pressure values in each air flow line in a desired operating range. However, due to the operational delay characteristic towards the signal response, conventional pressure adjustments can only be made at idle. As a result, it is difficult to obtain the pressure control on a real time basis, especially during the continuous rotational movements of wafer polishing. Such occurrences certainly assert unstable pressure conditions of the polishing, and possibly cause the wafer to depart from the wafer head to cause wafer damages.
Besides the ineffectiveness of process control, another drawback with conventional pneumatic monitoring system is the lengthy troubleshooting procedure. Generally, the monitoring procedure is carried out separately for the wafer air 30 and polish air 40 from the signal readings shown on the individually equipped transducer which senses the output pressure of the corresponding regulator. Due to the independencies of the monitoring circuits, in many instances, however, it is difficult to detect individual hardware (especially the transducer) problem during the operation merely from the signalled pressures (hardware problems would cause incorrect signals and false readings). Such occurrences thus make the system control problematic. The pressure drifts sometimes become undetectable unless visible false operation has been shown (such as the wafer falling off the wafer head). False operation could cause polishing process to shutdown and is usually in need of a troubleshooting procedure to resume desirable operation. However, such troubleshooting could be quite time consuming for there may not be clear clues for finding the faulty component of the false operation. In such cases, all the suspected parts, that includes the ones in wafer side, in pad side and the polished wafer self, have to be checked to assure their applicability.
Therefore, there is a need for an effective pressure monitoring system capable of accurately regulating air streams in a CMP process on a real time basis and practically reducing such tedious troubleshooting procedure so as to optimize and improve the CMP process.
It is an object of the present invention to provide a pressure monitoring system which eliminates the problems of the prior art system.
It is a further object of the present invention to provide a pressure monitoring system whereby air pressures are monitored on a real time basis with a close loop arrangement.
Yet another object is to provide a pressure monitoring system whereby the troubleshooting procedure of the system hardware is substantially reduced.
In accordance with the present invention, there is provided a system and method for monitoring pneumatic pressures to facilitate chemical mechanical polishing (CMP). The pressure monitoring system, arranged in a close loop circuit, includes an air regulator, a pressure transducer, a pressure difference transducer and a pressure difference regulator. In one embodiment, the hardware is equipped to facilitate findings of three control parameters of the monitoring systemxe2x80x94polishing air pressure (Pp), pressure difference of the polishing air pressure and a corresponding wafer air pressure (Dp), and deviation of the output pressure difference from a set point pressure difference (Cp).
By monitoring Pp, Dp and Cp, air streams in a CMP process can be effectively regulated on a real time basis and the troubleshooting procedure for the system hardware can be practically reduced.